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53. IodineName: Iodine
Iodine is a bluish-black, lustrous solid. It volatilises at ambient temperatures into a pretty blue-violet gas with an irritating odour. It forms compounds with most elements, but is less reactive than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulphide to form beautiful purple solutions. It is only slightly soluble in water. Iodine compounds are important in organic chemistry and very useful in medicine and photography. Lack of iodine is the cause of goitre (Derbyshire neck). The deep blue colour with starch solution is characteristic of the free element. It is assimilated by seaweeds from which it may be recovered, and is found in Chilean saltpetre, caliche, old salt brines, and salt wells. Iodine (Greek iodes meaning "violet") was discovered by Barnard Courtois in 1811. He was the son of a manufacturer of Saltpeter (potassium nitrate, a vital part of gunpowder). At the time France was at war and gunpowder was in great demand. Saltpeter was isolated from seaweed washed up on the coasts of Normandy and Brittany. To isolate the potassium nitrate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding sulfuric acid. One day Curtois added too much sulfuric acid and cloud of purple vapor rose. Curtois noted that the vapor crystallised on cold surfaces making dark crystals. Curtois suspected that this was a new element but lacked the money to pursue his observations. However he gave samples to his friends, Charles Bernard Desormes (1777-1862) and Nicolas Clement (1779-1841) to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac (1778–1850), a well-known chemist at that time, and to Andre-Marie Ampere (1775-1836). On November 29, 1813 Dersormes and Clement made public Curtois’ discovery. They described the substance to a meeting of the Imperial Institute of France. On December 6 Gay-Lussac announced that the new substance was either an element or a compound of oxygen. Ampere had given some of his sample to Humphrey Davy (1778-1829). Davy did some experiments on the substance and noted its similarity to chlorine. Davy sent a letter dated December 10 to the Royal Society of London stating that he had identified a new element. A large argument erupted between Davy and Gay-Lussac over who identified iodine first but both scientists acknowledged Bernard Curtois as the first to isolate the element. Direct contact with skin can cause lesions so care needs to be taken in handling iodine. Iodine vapor is very irritating to eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m3 (8-hour time-weighted average - 40-hour). Quick links
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